The design of components in rotor blades for rotorcraft, especially helicopters, is very challenging. The main rotor of a helicopter of standard design may be mounted for rotation about an approximately vertical axis passing close to the centre of mass of the helicopter. When the helicopter is not moving forwards through the air, the blades on the helicopter may experience approximately constant conditions as they rotate through an entire revolution about the vertical axis. As is well known, however, at other times, for example when the helicopter is moving forwards through the air, the conditions for a blade, especially its airspeed, may be very different when the blade is moving forwards on one side of the helicopter from when it is moving rearwardly on the other side of the helicopter. In order to take account of this problem a swashplate may be provided in the region of the connection of the rotor blades to the swashplate and, through a mechanical camming action, may alter a characteristic of each blade as it rotates; for example, the pitch of the blades may be varied.
In order to provide a control via a swashplate of the kind just described relatively large loads must be applied and this leads both to high energy consumption levels and to an increase in the mass of the helicopter. Similar situations arise with other forms of rotorcraft for example autogyros.
In an attempt to avoid such disadvantages and to provide more adjustability of the blade, it has been proposed to provide adjustable aerodynamic elements on a rotor blade in a manner somewhat similar to that in which elements might be provided on a fixed wing of an aircraft. There are, however, substantial additional complications if such an approach is adopted in respect of a rotor of a rotorcraft: a first principal difference is that the rotor rotates in use at considerable speed relative to the body of the rotorcraft making transmission of power from the body of the rotorcraft to the rotor more difficult; the second principal difference is that the rotor rotates at a high speed so that, especially towards the tip of the rotor blade, there may be high g forces. For example, it is not unusual for there to be g forces of 700 g in the region of a tip of a rotor blade.
It has been proposed to provide an actuator including an electric motor in a rotor blade in order to avoid transmission of mechanical power via the swash plate. The electric motor drives a rotary drive shaft and the rotation of that shaft then needs to be converted typically into a linear motion to drive an adjustable aerodynamic element on the rotor blade. It has been proposed to convert the rotary motion into linear motion by providing a travelling nut that is held against-rotation and screw threadedly engages a correspondingly threaded portion of the drive shaft.
A problem that arises in an actuator of the kind just described concerns lubrication of the actuator. This is a particular challenge because the high g forces that are typically present in a rotor blade act against natural circulation of lubricant to the parts that require lubrication. In an attempt to overcome this problem, WO2008/147450 proposes an arrangement in which an electric motor and drive shaft are immersed in a cavity full of lubricant. Since the cavity is full of lubricant there is no scope for lubricant to move to one end of the cavity so that all the parts within the cavity are exposed to lubricant. This approach has various disadvantages, however; a particular problem is that the lubricant inevitably creates resistance to relative movement of parts leading to considerable viscous losses. That is a particular issue in a conventional electric motor where the rotor is closely surrounded by the stator and the presence of lubricant can lead to high viscous losses, especially when the whole of the space between the armature and the stator is filled with lubricant.
It is an object of the invention to provide an improved actuator for use in a rotor blade of a rotorcraft and to provide an improved method of actuating an aerodynamic element on a rotor blade.